ENDOTHELIUM. 13 



EXAMINATION (H). Many blood-corpuscles will be seen. Neglecting these, study a 

 liver-cell. Notice its polygonal shape, yellowish granular protoplasm, and its nucleus deeply 

 stained red. No cell-envelope is visible. Sometimes there are fine granules of pigment in 

 the protoplasm, at other times fatty granules, especially if the liver of a stall-fed ox be 

 used. In young animals (dog) the liver-cells often contain two nuclei. The ' granular ' cha- 

 racter of the nucleus and protoplasm are due to the existence of a plexus -of fibrils, which 

 requires a higher magnifying power to make it obvious (PI. II., Fig. 12). 



The liver-cells of the newt are very large, and are easily prepared by placing small pieces 

 of the liver in a five per cent, solution of ammonium chromate, and after washing, staining 

 them with picrocarmine for several hours. When teased out in glycerine they show most 

 admirably the intra-nuclear plexus of fibrils. The liver of the newt, especially at the end of 

 winter, often contains particles of pigment (PL II., Fig. 12). 



ENDOTHELIUM. 



It is convenient to apply this term to a single layer of squamous epithelium wherever it 

 occurs, as the lining of serous sacs, blood- and lymph-vessels, &c. 



PEEPARATION BY THE SILVER PROCESS. Bleed a rabbit, take a portion of its omen- 

 turn and rinse it gently in distilled water to remove soluble chlorides. Place it in a quarter 

 per cent, solution of nitrate of silver for ten minutes or until it assumes a milky appearance. 

 Wash it thoroughly in ordinary water, and expose it to the action of diffuse sunlight, either in 

 water or in water and alcohol, until it assumes a brownish colour. Mount a small piece 

 unstained in Farrant's solution, and to compare with it, stain a similar piece with logwood for 

 ten minutes, and after washing away all the surplus logwood, mount it also in Farrant's solu- 

 tion. Picrocarmine does not stain so well after silver, but after a tissue has been in alcohol 

 for some time it takes up this pigment more easily. The omentum of the rabbit is chosen 

 because in it the endothelium occurs in a nearly continuous sheet. 



EXAMINATION (H). The tissue will be mapped out into a series of small polygonal 

 areas by narrow black lines ' silver lines.' These are produced by a deposit of the reduced 

 silver in the intercellular cement. The flattened epithelial cells adhere by their margins and 

 form a complete epithelium or endothelial investment for the connective tissue of which the 

 omentum consists. Focus carefully through the thickness of the membrane, and a second set 

 of lines, whose outlines do not correspond with those lying above them, will be brought into 

 view ; these are the endothelial cells on the other surface of the membrane (PI. II., Fig. 3). It 

 is important to practise this method of focussing through the whole thickness of an object. 

 Usually no nucleus is observable within the cells, but in the logwood preparation each cell will 

 be seen to contain an oval nucleus. Preparations of other serous membranes will be made 

 later on. 



